Silver (Ag) is a metal which is expected to be used in various industrial applications including electrode/wiring materials, binding materials/bonding materials, heat conduction materials, electrically conductive binding material/electrically conductive bonding materials, reflection coating materials, catalysts, and antibacterial materials because silver has excellent electrical conductivity and light reflectivity, and also specific properties such as a catalytic action and an antibacterial action. Use forms of silver in these various applications include dispersions/suspensions of silver particles in an appropriate solvent. For example, in formation of electrodes/wirings for wiring boards that are mounted on electronic components such as semiconductor devices, or in binding materials/bonding materials, electrically conductive binding materials/electrically conductive bonding materials and heat conduction materials, silver particles are formed into a paste, and the resulting metal paste is applied and fired to form a desired electrode, wiring, joint or pattern.
A liquid phase reduction method is generally known as a method for producing silver particles. In the method for producing silver particles by a liquid phase reduction method, a silver compound as a precursor is dissolved in a solvent, and a reducing agent is added to the solution to precipitate silver. At this time, a compound called a protective agent is generally added for inhibiting precipitated silver particles from aggregating to coarsen. The protective agent combines with reductively precipitated silver particles, and inhibits mutual contact of silver particles to prevent aggregation of silver particles.
A method for producing silver particles by a liquid phase reduction method can efficiently produce silver particles by adjusting the concentration of a silver compound in a solvent, and the type and added amount of a reducing agent, and properly selecting a protective agent. However, silver particles produced by the liquid phase reduction method tend to have a relatively large particle size, and have a wide particle size distribution due to the concentration gradient of reactants in a solvent. With silver particles having such a large particle size, electrodes and wirings with a size of several microns cannot be formed, and miniaturization of semiconductor devices etc. in recent years cannot be met. When silver particles having a wide particle size distribution are used, the film thickness tends to be uneven, and it is therefore difficult to use such silver particles in applications where smoothness is required.
As a method for producing silver particles, which replaces the liquid phase reduction method, a thermal decomposition method using a silver complex as a precursor is reported (Patent Document 1). This method basically makes use of the properties of a thermally decomposable silver compound such as silver oxalate (Ag2C2O4), and includes forming a complex of a silver compound and an organic compound serving as a protective agent, and heating the complex as a precursor to prepare silver particles. In Patent Document 1, an amine is added to silver oxalate as a protective agent to form a silver-amine complex, and the complex is heated at a predetermined temperature and thermally decomposed to produce silver particles.